In Colorado, between Denver and Pueblo, both the Denver and Rio Grande Western and Atchison Topeka & Santa Fe ran parallel single track lines, via Palmer Lake and Colorado Springs. The lines were built in the late 1800s.
For several decades now the two lines have been operating as a "Joint Line" with track sharing arrangements. In most segments the two lines run side-by-side, but in the Colorado Springs area they ran a few miles apart. The D&RGW ran on the west side of town, while the AT&SF ran through the center. (Burlington Route subsidiary Colorado & Southern also shared the AT&SF tracks, as it had abandoned its parallel route, further east, around World War II.)
Apparently Colorado Springs was one of several cities around the country that had a "No Whistling" ordinance, where trains were not allowed to sound the horn in the city limits. The AT&SF line had several grade crossings in town, and ran through some residential areas. Evidently train-car collisions happened too often, and the AT&SF was asked to "leave town". In 1972, the AT&SF did leave — it started running on the D&RGW on the west side.
The downtown AT&SF/C&S freight and passenger stations are still connected by a spur (the old main line), which connects with the active main lines a few miles south of town, near a place called Kelker. The tracks north of the stations were removed in 1972 for most of the 20-mile distance to Palmer Lake. (A short segment in North Colorado Springs is still in place, connected to the active Joint Line via a remnant of the old Chicago Rock Island & Pacific).
The photo below shows the Santa Fe station (now a business park). The station tracks ran through the area shown as a parking lot; the station is well-maintained but the new one story office buildings in front ruin the appearance. Most of the right-of-way through town is intact (not built on), and there is a jogging/bike path on part of it.
The photo above shows the old Rock Island heading east, with AT&SF bridge abutments in the distance. If you had been there in the late-60s, would you have believed the AT&SF track would disappear before the Rock Island? There is a junction/connector track east of the location of the old bridge, and the AT&SF track to the north (left side of bridge abutments, out of view) is still in place for a mile or so but in really poor shape.
The abandoned section of the ATSF ran from Palmer Lake to Monument, Pring, Sommers, Breed and into Colorado Springs. South of town, the abandonment extends from South Colorado Springs to Kelker, then a segment of the former D&RGW is abandoned from Kelker to Crews.
This article also includes pictures showing evidence of the Chicago, Rock Island and Pacific in Colorado Springs, CO.
Thanks to Mike Palmer for contributing information about this route.